Current Green Chemistry - Volume 7, Issue 1, 2020

The construction of carbon-carbon bond through the metathesis reactions between carbonyls and olefins or alkynes has attracted significant interest in organic chemistry due to its high atomeconomy and efficiency. In this regard, carbonyl–alkyne metathesis is well developed and widely used in organic synthesis for the atom-efficient construction of various carbocycles and heterocycles in the presence of catalytic Lewis acids or Brønsted acids. On the other hand, alkene-carbonyl metathesis is recently developed and has been a topic of great importance in the field of organic chemistry because they possess attractive qualities involving metal-mediated, metal-free intramolecular, photochemical, Lewis acid-mediated ring-closing metathesis, ring-opening metathesis and cross-metathesis. This review covers most of the strategies of carbonyl–alkyne and carbonyl–olefin metathesis reactions in the synthesis of complex molecules, natural products and pharmaceuticals as well as provides an overview of exploration of the metathesis reactions with high atom-economy as well as environmentally and ecologically benign reaction conditions.

Heterogeneous catalysis represents one of the important areas in the field of organic synthesis. Major developments have been emerged during last few decades and polymer-supported catalysts have been employed successfully in various catalytic organic transformations. Ion-exchange resins and polypeptides are two important examples of such heterogeneous polymer-supported catalysts among others because of their easy accessibility, stability, recoverability and reusability. Cross-linked ion-exchange resins and polypeptides are highly insoluble, which make them better choice in terms of their easy separation from the reaction mixture and subsequent recyclability. The present review article provides an overview of different types of ion exchange resins as polymer-supported catalysts such as amberlite resin, polystyrene resin, polyionic gel-based systems, ion-exchange resins and prolineimmobilized species, PEG-bound poly (amino acid), amino acid anchored with Merrifild resin, amphiphilic block polypeptides etc. Their preparation, characterizations and catalytic applications in diverse organic transformations have been presented with critical analysis on their stability, mechanistic overview and suitability etc.

Nanoparticle catalyzed synthesis is a green and convenient method to achieve most of the chemical transformations in water or other green solvents. Nanoparticle ensures an easy isolation process of catalyst as well as products from the reaction mixture avoiding the hectic work up procedure. Zinc oxide is a biocompatible, environmentally benign and economically viable nanocatalyst with effectivity comparable to the other metal nanocatalyst employed in several reaction strategies. This review mainly focuses on the recent applications of zinc oxide in the synthesis of biologically important heterocyclic molecules under sustainable reaction conditions. Application of zinc oxide in organic synthesis: Considering the achievable advantages of this nanocatalyst, presently several research groups are paying attention in anchoring zincoxide or its modified structure in several types of organic conversions e.g. multicomponent reactions, ligand-free coupling reactions, cycloaddition reaction, etc. The advantages and limitations of this nanocatalyst are also demonstrated. The present study aims to highlight the recent multifaceted applications of ZnO towards the synthesis of diverse heterocyclic motifs. Being a promising biocompatible nanoparticle, this catalyst has an important contribution in the fields of synthetic chemistry and medicinal chemistry.

Task-specific ionic liquids (TSILs) have received increased attention over the past few years as a Green Catalysts and Solvents for a large number of organic transformations. The present review article aims to provide an introduction, types of task-specific ionic liquids, preparation/synthesis, physical properties, characterization, use of TSILs as solvent and catalyst in organic synthesis.

Conformationally locked 3'-azido-C-4'-spirooxetano-xylonucleosides T, U, C and A have been synthesized by following chemo-enzymatic convergent route. One of the 3'-azido-C-4'- spirooxetano-xylonucleosides, i.e. T was converted into 3'-amino-C-4'-spirooxetano-xylothymidine by reduction of azide to amine with H2/Pd-C in ethyl acetate in quantitative yield. The crucial step in the synthesis of spirooxetano-xylonucleosides is the Lipozyme® TL IM-mediated exclusive diastereoselective acetylation of 4-C-hydroxymethyl group in dihydroxysugar derivative, 3-azido-3-deoxy-4-Chydroxymethyl- 1,2-O-isopropylidene-α-D-xylofuranose in quantitative yield. The diastereoselective monoacetylation of 4-C-hydroxymethyl in dihydroxysugar derivative was unambiguously confirmed by X-ray crystal study on the tosylated compound obtained by the tosylation of Lipozyme® TL IM - mediated monoacetylated sugar derivative. The broader substrate specificity and exclusive selective nature of Lipozyme® TL IM can be utilised for the development of environmentally friendly methodologies for the synthesis of different sugar-modified nucleosides of importance.

A simple, facile, straightforward and environmentally benign protocol has been developed for the efficient synthesis of pharmaceutically interesting 3,3-bis(indol-3-yl)indolin-2-ones, bis(indol- 3-yl)(aryl)methanes and tris(indol-3-yl)methanes using a catalytic amount of mandelic acid as an efficient, naturally occurring, low-cost, commercially available organo-catalyst in aqueous ethanol at room temperature.